The present invention is related to planet carriers used in planetary gear systems, and in particular to planet carriers used in wind turbine gearboxes.
In a planetary gear system, the function of a carrier is to transmit torque or torsion load from the input shaft into the planet pins, as evenly as possible. Particularly in wind turbine applications, the carrier is also transmitting the weight of the gearbox back to the wind turbine rotor shaft. The most efficient way of doing this is a cylinder, but a planet carrier must have gaps for the planet gears. The gaps cause the shear to be concentrated in the legs. Flexibility of the planet carrier is critical to the alignment of the planet gear meshes.
These loads can cause deformation of the carrier, resulting in one or more of the following:                Misalignment of the gear stage;        Additional loads applied to the gearbox, due to overcoming the active range of the gearbox mountings; and        Excessive vibration.        
Approaches to increasing the stiffness of the carrier to avoid these problems have included:                Thickening the carrier walls; and        Adding ribs        
However, these approaches introduce further problems.
For example, thicker walls add weight.
Use of ribs can cause local stress raisers, which can cause fatigue failures. This is more of a problem in brittle materials, such as castings. Ribs also make for a more complex casting.
Excessive wind-up (where the downwind ends of the planet pins rotate relative to the upwind ends of the pins, about the central axis of the carrier) causes the planet pins to tilt, which in turn causes misalignment in the planet gear mesh. Carrier wind-up can also cause an imbalance of loading between the upwind and downwind planet bearings. Twisting of the pins in the bores in the carrier can cause excessive stress in the carrier (or pin), resulting in local yielding or fatigue failure.
Approaches to addressing these problems have included:                Increasing the diameter of the planet pin interface with the carrier;        Increasing the interference fit of the planet pin interface with the carrier;        Shortening the length of the legs of the carrier;        Increasing the thickness of the legs of the carrier;        Changing the material of the carrier to a material having higher stiffness;        Increasing the diameter of the carrier; and/or        Increasing the thickness of the carrier plates.        
However, there is a limit to the space available within the planet carrier. Increasing the pin diameter can cause assembly problems (e.g. carrier bearing seats may have channels cut into them to allow wider pins to be fitted—this reduces bearing life).
Increased interference fit causes greater stress in the region, increasing risk of yield or fatigue problems. Tighter fits also make assembling and disassembling more costly and risks of damage during assembly increase.
Shorter legs mean a thinner gear, which reduces the load carrying capacity or life of the gear.
Increasing the thickness of legs of carrier may only be possible if there is space between the planet gears to do so. Reducing the planet gear size reduces the ratio change of the planet stage. Thicker legs increase the weight of the carrier.
A change to a stiffer material would add cost in raw material, and may require more complex casting procedures (e.g. change from SG iron to cast steel)
Increasing the diameter of the carrier is only possible if there is space in the ring gear. An increase in ring gear size would result in a change in the overall package size of the gearbox.
Increasing the thickness of the plates results in a great increase in weight.
As mentioned above, the role or function of the planet carrier is to transmit torque loads and, particularly for wind turbine application, transmit the weight of the gear box.
Approaches to addressing this shortcoming include those illustrated in FIGS. 7 and 8. Referring to FIG. 7, which shows planet carrier 700 having a shaft 702, and which is supported by bearing 704 on the downwind side of the gearbox and by a very large bearing 706 on the upwind side. Large bearing 706 is able to pass over the input flange. However, large bearings are expensive, and as the carrier bearings are usually lightly loaded, a larger bearing further reduces the loading, with a concomitant increase in the risk of skidding failure in the bearing.
Referring now to FIG. 8, which shows planet carrier 700 having a shaft 702 supported by bearing 704 on the downwind side of the gearbox, smaller diameter support 706 can be used by splitting the carrier or the upwind bearing. Split bearings are unlikely to be reliable, and the use of a split carrier requires an additional joint, at a smaller size than that with the rotor shaft, which would be very highly loaded, difficult to manufacture and add cost.
As can be seen in FIGS. 7 and 8, planet carrier 700,800 is supported on the non-rotor side of the carrier by bearing 704,804 to provide the necessary radial support.